Sealing mechanisms are provided around a rotational axis in a gas turbine to prevent gas from leaking to the low-pressure side from the high-pressure side, for example.
Labyrinth seals, which are non-contact seals, have traditionally been used for these sealing mechanisms, but as labyrinth seals make no contact, there are limits to how much their sealing performance can be improved. Accordingly, brush seals are now being used to achieve greater sealing performance.
A brush seal comprises multiple bristles formed into a ring shape (brush seal bristles), and the tips of these brush seal bristles contact the rotating side to provide a seal.
The rotating side, such as a rotor in a gas turbine, deforms due to centrifugal force and heat as the turbine operates, while the housing, which is the stationary side, also expands due to heat, and consequently the distance between the brush seal and rotor changes.
This distance change is absorbed by the deflection of bristles, but since deflection of bristles toward the low-pressure side can cause the sealing performance to drop, a back plate is provided on the low-pressure side of brush seal bristles to prevent them from deflecting toward the low-pressure side. Additionally, a space is provided on the high-pressure side of bristles to allow for deflection of bristles. (Refer to FIGS. 8 to 10 on Page 10 of Patent Literature 1, for example.)
If a brush seal is used in a high-pressure area in a steam turbine, etc., however, its bristles will flap, especially on the high-pressure side, and eventually break.
If bristles break, the broken bristles will no longer contact the rotor, thereby causing the sealing performance of the brush seal to drop.
A drop in the sealing performance of the brush seal can lead to a problem of lower output from the turbine, for example.
To eliminate the flapping of bristles on the high-pressure side, solutions are proposed that involve providing a braking plate also on the high-pressure side of brush seal bristles to restrict the deflection of brush seal bristles, while providing through holes in the braking plate to suppress generation of turbulent flows and particularly prevent the bristles from flapping on the high-pressure side. (Refer to Patent Literature 2, for example.)
However, the solution described in Patent Literature 2 is such that the movement of brush seal bristles is strongly restricted all the way until near the rotor by the back plate on the low-pressure side and braking plate on the high-pressure side, resulting in a shorter free deflection length of brush seal bristles.
This increases the rigidity of brush seal bristles and raises their contact pressure with the rotor, which in turn presents a problem of increased wear of the bristles and rotor in a steady state.
To solve the problems of the solution described in Patent Literature 2, solutions are proposed that involve providing restraining bristles having just enough elasticity not to prevent the deformation of the bristle part upon contact, in place of a braking plate on the high-pressure side of bristles. (Refer to Patent Literature 3, for example.)
The brush seal described in Patent Literature 3 absorbs any change in the interval between the stationary part and the rotational part by the deflection of the bristle part, and because the movement of the bristle part is suppressed by the back plate on the low-pressure side, these bristles deflect in the direction of the restraining bristles; since the restraining bristles have just enough elasticity not to prevent the deformation of the bristle part upon contact; the restraining bristles undergo elastic deformation and thereby absorb the deformation of the bristle part, and consequently any increase in wear can be prevented in a steady state without increasing the contact friction force as the interval between the stationary part and rotational part changes.
However, the brush seal described in Patent Literature 3 presents a problem of high manufacturing cost because restraining bristles formed by bundled wires (bristles) must be provided on the high-pressure side of the bristle part.